Pretty sure this has been covered in some form or fashion before...What is involved in installing a 67-72 steel bed floor in a 63 -66 fleetside? I know I need a spacer for the front panel , but are the early wheel tubs a bolt in?

I'm in the middle of doing this conversion on my '64. It will need a little trimming at the tailgate end. I'm making a spacer to mate up the original header panel to the floor, moswt people report using marine grade plywood.